Basal melt of ice shelves may lead to an accumulation of disc-shaped ice platelets
underneath nearby sea ice, to form a sub-ice platelet layer. Here we present the seasonal cycle of sea
ice attached to the Ekström Ice Shelf, Antarctica, and the underlying platelet layer in 2012. Ice platelets
emerged from the cavity and interacted with the fast-ice cover of Atka Bay as early as June. Episodic
accumulations throughout winter and spring led to an average platelet-layer thickness of 4m by
December 2012, with local maxima of up to 10 m. The additional buoyancy partly prevented surface
flooding and snow-ice formation, despite a thick snow cover. Subsequent thinning of the platelet layer
from December onwards was associated with an inflow of warm surface water. The combination of
model studies with observed fast-ice thickness revealed an average ice-volume fraction in the platelet
layer of 0.25+-0.1. We found that nearly half of the combined solid sea-ice and ice-platelet volume in
this area is generated by heat transfer to the ocean rather than to the atmosphere. The total ice-platelet
volume underlying Atka Bay fast ice was equivalent to more than one-fifth of the annual basal melt
volume under the Ekström Ice Shelf.